With the development of display technology, Liquid Crystal Display (LCD) and other flat display devices are widely used in various electronic products such as mobile phone, TV, personal digital assistant, digital camera, laptop, desktop computer and so on due to advantages of high image quality, power saving, a small thickness, and a wide usage range. LCD has become a mainstream display device.
At present, most of the LCDs in the market belong to a backlight-type LCD, which comprises a liquid crystal display panel and a backlight module. An operating principle of the liquid crystal display panel is as follows: liquid crystal molecules are filled between two glass substrates which are parallel to each other; a plurality of fine wires are arranged between the two glass substrates in vertical direction and in horizontal direction; voltage is applied to the liquid crystal molecules so that the liquid crystal molecules can deflect; and light that is emitted by the backlight module can be refracted and an image can be displayed on the panel.
The liquid crystal display panel generally comprises a color filter substrate, a Thin Film Transistor (TFT) substrate, Liquid Crystal (LC) filled between the color filter substrate and the TFT substrate, and a sealant.
The color filter substrate and the TFT substrate of an LCD both are provided with a thin film material, a main role of which is enabling the liquid crystal molecules to be aligned in a certain direction. The thin film is called as alignment film and is generally made of polyimide (PI) material. The alignment film is mainly made of rubbing alignment PI material and optical alignment PI material. However, these two kinds of alignment materials each have their respective disadvantages. As to the rubbing alignment PI material, dust particles, electrostatic residue, brush marks and other problems would possibly be resulted in, and a qualified rate of the product would be reduced. If the optical alignment PI material is used, the above problems can be solved. However, the optical alignment PI material has a poor heat resistance and aging resistance, and a weak ability to anchor LC molecules, and thus a quality of the panel will be affected. Moreover, the PI material itself has a high polarity and a high water absorption, and properties thereof would be easily changed during storage and transportation procedures, which will result in non-uniform alignment of the liquid crystal. Furthermore, the PI material is expensive, and the deposition procedure of the PI material on a TFT-LCD is complicated, which will increase a cost of the panel.
Since the PI material has properties of uniformity, binding ability, and stability, in addition to the role of enabling the liquid crystal molecules to be aligned, the PI material can also serve as an insulating film between an upper substrate and a lower substrate of the liquid crystal display panel. In this manner, during one frame period, a voltage applied to the upper substrate and the lower substrate can be maintained, so that ions and other impurities in the two substrates can be prevented from diffusing into the liquid crystal, and a quality of the liquid crystal will not be affected.
If the PI film in a TFT-LCD is omitted, the liquid crystal molecules cannot be aligned in the vertical direction. There is a method to omit the PI film in a TFT-LCD, i.e., to add a vertical alignment agent to the liquid crystal. The surfactant can be adsorbed on a surface of an ITO substrate, and thus the liquid crystal molecules can be aligned in the vertical direction. The liquid crystal material containing the vertical alignment agent is called as a self-alignment liquid crystal material.
However, if the PI film is completely omitted, on the one hand, problems such as mura and poor liquid crystal alignment would occur on a panel, and on the other hand, an afterimage of the panel would become more serious.